subsea

Upper Completion Components Subsea Engineering

Topics • Overview of Upper completion components • • • • •

Christmas Trees Well Head Tubing Hangers Tubing Packers and Seals

What is a Christmas Tree?

A subsea Christmas tree is basically a stack of valves installed on a subsea wellhead to provide a controllable interface between the well and the production facilities.

What are their functions? • The Christmas tree is the interface between the completion and surface facilities • It provides a pressure barrier between the well and the environment • Allows control of produced/injected fluids • Providing access to the annulus for well control, pressure monitoring, gas lift, etc. • Allows access to the completion for well intervention • Providing a hydraulic interface for the down hole safety valve • Providing an electrical interface for down hole instrumentation, electric submersible pumps, etc. • Providing structural support for BOPs, flow line and control umbilical interface.

Main Types of Christmas Trees • Main types of Christmas trees are : • Surface trees – Installed onshore or on platforms • Subsea trees – Located at the sea bed

Surface or Subsea?

Surface Tree • Solid block or flanged type • Manually operated

Flanged surface tree

Surface Tree (Continued)

Subsea Christmas Tree • Derived from a surface tree • Horizontal or Vertical • ROV Operated

Classification of Subsea Trees • Vertical Subsea Trees (Conventional) • Horizontal Subsea Trees

Subsea Tree Components The Subsea Christmas tree is a complex engineered system of components. There are several different types of trees as explained below, and the tree configurations available even within a given type of tree (e.g. horizontal tree, or dual bore tree) vary widely from project to project.

Subsea Tree Components- cont’d A Subsea Christmas tree will typically consist of the following components: • A tree connector to attach the tree to the subsea wellhead • The tree body, a heavy forging with production flow paths, designed for pressure containment. Annulus flow paths may also be included in the tree body. • Tree valves for the production bore, the annulus, and ancillary functions. The tree valves may be integral with the tree body or bolted on • Control system. This includes the valve actuator command system and includes pressure and temperature transducers • Choke (optional) for regulating the production flow rate. • External tree cap for protecting the upper tree connector and the tree itself. Tree cap often incorporates dropped object protection or fishing trawl protection.

Subsea Tree Valves 

Master Valve - A valve located on the Christmas Tree that controls all flow from the wellbore. A correctly functioning master valve is so important that two master valves are fitted to most Christmas trees. The upper master valve is used on a routine basis, with the lower master valve providing backup or contingency function in the event that the normal service valve is leaking and needs replacement.

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Swab Valve - The topmost valve on a Christmas tree that provides vertical access to the wellbore. It lies in the path used for well intervention like wireline and coiled tubing

Subsea Tree Valves cont’d • Wing Valve - A valve located on the side of a Christmas tree or temporary surface flow equipment, such as may be used for a drill stem test. Two wing valves are generally fitted to a Christmas tree. A flowing wing valve is used to control and isolate production , and the kill wing valve fitted on the opposite side of the Christmas tree is available for treatment or well-control purposes. The term wing valve typically is used when referring to the flowing wing. • Crossover Valve - A crossover valve allows circulation of the riser after disconnection via the EDP (Emergency disconnect Package)

Vertical Subsea Tree

Dual Bore configuration

Tubing hanger landed in wellhead.  Two production master valves may be provided.  The lower production master valve is designed for ROV operation.  There is usually only one annulus master valve.

Features of the Dual bore Vertical Trees  The completion tubing is landed in the wellhead housing.  Two vertical bores exist through the tree allowing access to the tubing and tubing/production casing annulus.  Each bore contains a master valve and a swab valve, so providing two independent barriers to flow.  Work-over is carried out with a dual bore lower riser package, emergency disconnect package and riser.  Most trees can be run and recovered with ROV intervention.

Horizontal Subsea Tree Features

Tubing hanger landed in tree.

Vertical Subsea Tree Features  Well completion tubing is landed in the tree.  Valves are external providing a clear path to the well bore.  An internal tree cap is run on top of the tubing hanger in which crown plugs can be set by wireline.  The large through-bore design of a horizontal tree allows the installation and retrieval of downhole equipment through the tree, including artificial lift completions, without having to remove the tree or disconnect flowlines.  Most trees can be run and recovered with assistance of ROV intervention.  Horizontal trees are run and retrieved on drill pipe.  Stack up height is less than on a horizontal tree, therefore there is less risk of damage due to trawl boards etc.

Horizontal Tree with ROV docking plate

Pressure and Structural Design Considerations for Trees • Pressure containing components of subsea trees are to be designed and tested in accordance with API 17D and API 6A for pressure ratings of 5000, 10000 and 15000 psi for most applications • The tree piping is normally designed in accordance with ASME B31.3 • The guidelines in the API specifications are general and in many case open to interpretation. It is up to the manufacturer to apply his engineering judgment.

Designing for pressure in Trees The sources of pressure in a subsea tree include the following: • Production fluids. • Hydraulic fluid. The hydraulic fluid pressure to the SCSSV may exceed the tree pressure rating. Effects of primary seal failures should be considered. • Chemical injection fluids. Seal failures can result in migration of fluids • Thermal expansion of fluids in closed cavities. • Annulus pressure. It should be assumed that pressure will accumulate in the well annulus. • External hydrostatic pressure. • Test pressure. Seal verification pipeline • Hydraulic lock. When mating parts are engaged, fluids may become trapped in the enclosed cavity and impede the engagement of the parts or cause damage to some component.

Structural Design Considerations Trees The tree connector, tree body, tree guide frame and tree piping must be designed to withstand internal and external structural loads imposed during installation and operation. The following are some tree and tree component load considerations: • Riser and BOP loads. • Flowline connection loads. • Snagged tree frame, umbilicals or flowlines. • Thermal stresses – trapped fluids, component expansion, pipeline growth. • Lifting loads. • Dropped objects. • Pressure induced loads – external and internal.

Subsea Tree Installation

First things First… The sequence of operations for installing a subsea tree onto a predrilled well: • Move the rig onto location. • Launch ROV to locate the wellhead. • Establish final position over wellhead with the aid of the ROV and drill string reference positioning system. • Retrieve the corrosion cap from the wellhead. Check the condition of wellhead sealing surface with the ROV and flush if necessary. • Verify that a wellhead wear bushing is not in place. If it is, it will need to be retrieved. This is normally done through the BOP stack after it is run. For a horizontal tree, it may be retrieved in open water to avoid having to trip the BOP stack for just the wear bushing retrieval

Installing a Horizontal Tree (1)  Horizontal trees are thought of as driller friendly trees.  Once the production liner or casing has been cemented in place and pressure tested (first barrier), a second barrier, such as a retrievable test tree packer, is installed +/- 500 ft below the wellhead and pressure tested.  The BOPs can now be unlatched, the rig skidded to a safe area and the BOPs retrieved on the drilling riser and moved to the stump.  The tree is then moved to the moonpool, function tested and connected to drill pipe with a tree running tool (TRT).  The tree is then run to just above the seabed, the rig skidded back over the wellhead and the tree lowered and latched on with the aid of a control umbilical strapped to the drill pipe.

Installing a Horizontal Tree (2)  The tree is then function and pressure tested before the TRT and umbilical are retrieved.  The rig’s BOP and riser is then run and latched onto the profile at the top of the tree.  After testing the BOP and tree connection, the downhole packer is removed to enable the completion to be run and landed in the tree.  Note 1. A dedicated marine riser is not required and the rig’s own safety equipment is employed.  Note 2. A dropped BOP is a major concern as more BOP runs are required to install a horizontal tree. This risk is mitigated to a degree by skidding the rig to a safe handling area.

Installing a Vertical Tree • The BOP stack is run onto the wellhead before any completion work commences • The tubing hanger is run and oriented with the installation and work over riser configured for the tubing hanger running tool • The tree is then run on the installation and work over riser configured for the tree running tool and lower marine riser package • The riser is then retrieved and a debris cap run onto the tree.

Flow Base For Vertical Tree

The provision of a flow base overcomes the problem of having to disconnect flowlines from vertical trees prior retrieval in preparation for work-over operations.

Consideration for Tree selection  The cost of a horizontal tree can be 20 – 40% less than a vertical tree.  Changing out horizontal trees is more time consuming than changing out a vertical tree, since completions must be retrieved first.  When completion operations immediately follow drilling operations, two BOP trips are required to install a horizontal tree and only one to install a vertical tree.  For wells requiring several completion changes during the life cycle of a well, a horizontal tree will save time since it remains in place and flow lines need not be disconnected.  The integrity of a horizontal tree depends on the reliability of the metal to metal seals used at the tree/tubing hanger interface and the pipes and valves external to the tree.  Modern vertical trees are connected to a flow base which allows the flowlines to remain connected when recovering the tree.

Overview of Upper completion components • • • • •

Christmas Trees Well Head Tubing Hangers Tubing Packers and Seals

Well Head and its functions • Wellheads transfer the loads from the casing and the completion to the ground via the surface casing • It provides a pressure barrier and flow/access control to the annular (all of them) and the tubing string • API specification 6A is used as a guide for wellhead design • From a completion standpoint, loads are transferred from the completion string to the wellhead via the tubing hanger

Well Head and its functions – Cont’d  In subsea wellheads, each casing hanger is landed within the high pressure wellhead, stacked one on top of the other, smaller above larger.  Each hanger is equipped with a metal to metal seal designed to isolate the casing annulus from wellbore pressure.  Subsea wellheads provide structural and pressure integrity for the casing and tubing strings that go to make up an oil and gas well.

Establishing a Subsea Well

Temporary guide base is run on guidelines and 36” hole drilled.

Retrieve running tool.

RIH with conductor, LP wellhead and permanent guide base, cement conductor.

Drill 26” hole, run 20” casing with HP wellhead, land in LP wellhead, cement 20” casing.

Onshore - Wellhead

Selah Pit

Wellhead Selection Issues • Completion loads will be transfer via the tubing hanger • Annular injection requirements (P. test, gas lift..) will dictate orifice and valve sizes • “Wet” parts of the wellhead might require different metallurgy • Instrumentation and data acquisition requirements

Overview of Upper completion components • • • • •

Christmas Trees Well Head Tubing Hangers Tubing Packers and Seals

Tubing Hangers • Transfers loads from the completion to the ground via the surface casing • Structural support to tubing • Pressure and flow barrier

Types of Tubing hanger There are five (5) types • Mandrel type • Ram tension type • Slip and seals • Direct suspension • Sub-mud line type

Tubing hanger selection • Mechanical rating (loading, pressures) • Number of bores(flow paths) and orifices(cables, penetrators, hydraulic conduits..) • For subsea trees, annular access requires an additional bore • Operational considerations, landing procedure…

Overview of Upper completion components • • • • •

Christmas Trees Well Head Tubing Hangers Tubing Packers and Seals

Tubing Design The cost of tubulars and completion components is often a large portion of the total well cost and can be as high as 20% of the total. The tubing and completion forms an integral part of the safety structure of the well. Failure of the completion can result in injuries, fatalities, major expenditure and considerable loss of production

Tubing Design – cont’d Tubing strings are designed based on the material elastic behaviour, key properties are: • Tensile stress • Yield stress • Young modulus • Poisson ration • Ultimate tensile stress

Stress – Strain Behaviour

Elastic Behavior of steel

Plastic Behavior

Mechanical properties of materials

Mechanical Loads in tubing

Mechanical Design for tubing strings

Axial Forces - Weight

Buoyancy Effect

Thermal Loading

Internal and External Pressures

Slack-off and pull of forces

Buckling

Burst Loading

Collapse Loading

Triaxial Design

Modeling Triaxial Stresses

Representation of the results

Other Considerations

Final Loading Diagram